My idea is a device that gives its user great power over the ocean, its currents, and presumably temperatures. Could this be used to dramatically (and in relatively short time) effect the climate of a region near the ocean, causing a widespread freeze?

Alternately, could a device that has power over weather patterns then be used to affect the ocean, such as changing currents or causing widespread flooding?

$\begingroup$Of course. Both options are valid provided the devices are magical enough. The first device is known as Alexias; all the user has to say is "Alexias, Oceanum congela!" and the oceans will be frozen solid, from the depth of the deepest trench to the shores of the warm seas. The second device is known as Quart-Anna. At the command "Quart-Anna, let the Ocean flood them!" the waters of the Ocean River will move uphill unto land, causing untold flooding, death and destruction.$\endgroup$
– AlexPMar 2 '19 at 17:42